Thermodynamic model of MSWI flue gas cooling path: Effect of flue gas composition on heavy metal binding forms.

In the context of circular economy and heavy metal (HM) recovery from municipal solid waste incineration (MSWI) fly ash (FA), detailed knowledge of HM binding forms is required for achieving higher extraction rates. The FA mineralogy is still poorly understood due to its low grain size and low metal concentration. To investigate the HM binding forms, a sophisticated thermodynamic reactive transport model was developed to simulate ash-forming processes. The stability of different binding forms was investigated at different flue gas conditions (varying ratios of HCl, SO2, O2) by simulating the gas cooling path in closed system and dynamic open system, where the gas composition is changing upon cooling due to precipitation of solids. The simulations predict that at flue gas conditions of molar ratio S/Cl < 1, Cu and Zn precipitate as oxides (and Zn silicates) at approximately 650°C. At temperatures <300°C, Zn, Cu, Pb and Cd are predicted to precipitate as easily soluble chlorides. In flue gas with molar ratio S/Cl > 1, the HM precipitate as less soluble sulphates. The results indicate that the less soluble HM fraction in the electrostatic precipitator ash represent oxides and silicates that formed in the boiler section but were transported to the electrostatic precipitator. The model provides insight into the physical-chemical processes controlling the metal accumulation in the flue gas and FA during the cooling of the flue gas. The obtained data serve as valuable basis for improving metal recovery from MSWI FA.

Effects of enhanced metal recovery on the recycling potential of MSWI bottom ash fractions in various legal frameworks.

In recent years, complex new bottom ash treatment processes for enhanced metal recovery have been implemented in Switzerland, producing residual bottom ash fractions with various qualities. This study focusses on three different treatment processes by characterizing all arising fractions in detail. Thereby the factors influencing the composition of these fractions are identified and their recycling potential in Switzerland is investigated. However, high legislative requirements on total contents of heavy metals represent a high barrier for bottom ash recycling in Switzerland. Therefore, the recycling potential is further evaluated based on the waste legislation applied in the Netherlands, where recycling of bottom ash has a long tradition. There, threshold values for bottom ash recycling are based on leachate concentrations and not on total contents as in Switzerland. However, Swiss Waste Legislation also knows threshold values based on leachate concentrations for certain waste materials. The leaching tests applied in these two countries, however, are different. The comparison of both leaching tests reveals that the setup and conditions, especially the considered pH range, significantly influence the leaching of heavy metals. With emphasis on problematic pollutants, the possibilities for new applications of these fractions are evaluated based on Swiss and Dutch legal threshold values. The comparison within the legal frameworks of these two countries allows recognizing opportunities and risks related to bottom ash recycling.

Dataset on ten-years monitoring of MSWI bottom ashes in six MSWI plants in the Canton of Zürich, Switzerland.

The dataset presented in this article is the supplementary data for the research article "Ten-years monitoring of MSWI bottom ashes with focus on TOC development and leaching behaviour" (https://doi.org/10.1016/j.wasman.2020.07.038) by Glauser et al. (2020) [1]. From 2008-2018 bottom ashes have been monitored in six MSWI plants in the Canton of Zürich with regular sampling campaigns and analysis of important species defined in the Swiss Waste Legislation [2]. Both the size of the dataset and the long period of consistent and representative monitoring are unique for Switzerland. Relevant aspects of the monitoring data are discussed and interpreted in the above mentioned research article and complemented by simple emission forecast modelling. While only selected species were discussed in the research article, this data article covers all the monitoring data. The focus of the monitoring was laid on carbon-species with the analysis of total carbon (TC), total organic carbon (TOC), total inorganic carbon (TIC), degradable organic carbon (OC) and elemental carbon (EC). Total contents of nitrogen (N), sulphur (S), phosphorus (P), selected heavy metals (As, Cd, Cr, Cu, Ni, Pb, Sb and Zn) and loss on ignition (LOI) complete the solid chemical analysis. In addition, particulate ferrous (Fe) and non-ferrous (NF) metals and unburnt material were determined manually. Batch eluate tests according to Swiss Waste Legislation [3] were performed and analysed for dissolved organic carbon (DOC), ammonium (NH4 +), nitrite (NO2 -), fluoride (F-), sulphite (SO3 2-), sulphide (S2 -), chromate Cr(IV) and the heavy metals Cu (aq) and Zn (aq) and Cr(IV). In addition, data on the biochemical oxygen demand (BOD) and the physical parameters pH and electrical conductivity complete the eluate analysis.

Ten-years monitoring of MSWI bottom ashes with focus on TOC development and leaching behaviour.

In Switzerland MSWI bottom ash has to comply with the legal threshold value for TOC of < 2 wt-% in order to be landfilled. However, TOC contents of this magnitude lead to elevated DOC emissions and associated emissions of ammonium and Cu (aq). Since 2008 the Canton of Zürich therefore pursues a strategy to lower TOC contents in bottom ash by 2020 to 0.5 wt-%. To observe the development of TOC and other constituents, bottom ash has been monitored from 2008 to 2018. Monitoring results indicate that TOC contents < 0.5 wt-% in bottom ash lead to DOC eluate concentrations < 20 mg/l. DOC concentrations of this magnitude are close to Swiss legal criteria for discharge of landfill leachate into surface waters (10 mg/l). The emission results have been obtained by batch eluate tests according to Swiss Waste Legislation. Such laboratory tests only partially simulate real conditions occurring on landfills. To approximate landfill conditions, column tests with recent bottom ashes combined with tests on simple emission forecasting complete the study. The comparison of results from batch and column tests shows similar cumulative concentrations, indicating that batch tests are suitable to evaluate bottom ash quality. The tested modelling approach, based on constant conditions and exponential decrease in concentration, proved adequate to simulate column progressions. The modelled emission forecasts for DOC lies within 33% of column test results. Further, the model demonstrates the differences in flow regime between eluate tests and landfills and promotes better understanding of temporal aspects and the influence of landfill relevant parameters on pollutant mobilisation.

Predicting Phosphate Release from Sewage Sludge Ash Using an Ion Sink Assay.

Thermochemical treatments allow production of sewage sludge ash (SSA) rich in P and low in heavy metals, which could be recycled in agriculture. Our objective was to quantify P release from SSA using ion sink assays and to relate these results to P speciation in SSA and plant P uptake. Anion and cation exchange membranes saturated with different counterions (HCO, Na, and H) were used to create a gradient in pH, P, or cation concentration between SSA particles and the surrounding solution. Phosphorus speciation in SSA was assessed using X-ray powder diffraction, and plant P uptake was determined in a pot experiment with an acidic and a neutral soil. Four SSA products were investigated: a SSA thermochemically treated with CaCl or MgCl (SSA Ca/Mg), a SSA blended with KCl, and a SSA blended with KCl and triple superphosphate (TSP) to obtain a marketable 12-20 P-K fertilizer. The H membranes dissolved all P species present in SSA. Combined HCO/Na membranes extracted diffusible P and noncrystalline P from SSA Ca/Mg and stanfieldite from SSA Mg. Blending with KCl hardly changed P release from SSA, whereas blending with TSP masked P release. The amount of P extracted from SSA by combined HCO/Na membranes was correlated to plant P use in the acid soil, whereas the amount of P extracted by HCO membranes alone was correlated to P use in the neutral soil. In conclusion, the ion sink assays delivered information on P release that was related to both SSA mineralogy and P use by plants.

Extraction of heavy metals from MSWI fly ash using hydrochloric acid and sodium chloride solution.

Fly ash from municipal solid waste incineration contains a large potential for recyclable metals such as Zn, Pb, Cu and Cd. The Swiss Waste Ordinance prescribes the treatment of fly ash and recovery of metals to be implemented by 2021. More than 60% of the fly ash in Switzerland is acid leached according to the FLUWA process, which provides the basis for metal recovery. The investigation and optimization of the FLUWA process is of increasing interest and an industrial solution for direct metal recovery within Switzerland is in development. With this work, a detailed laboratory study on different filter cakes from fly ash leaching using HCl 5% (represents the FLUWA process) and concentrated sodium chloride solution (300 g/L) is described. This two-step leaching of fly ash is an efficient combination for the mobilization of a high percentage of heavy metals from fly ash (Pb, Cd ≥ 90% and Cu, Zn 70-80%). The depletion of these metals is mainly due to a combination of redox reaction and metal-chloride-complex formation. The results indicate a way forward for an improved metal depletion and recovery from fly ash that has potential for application at industrial scale.

Chemical associations and mobilization of heavy metals in fly ash from municipal solid waste incineration.

This study focusses on chemical and mineralogical characterization of fly ash and leached filter cake and on the determination of parameters influencing metal mobilization by leaching. Three different leaching processes of fly ash from municipal solid waste incineration (MSWI) plants in Switzerland comprise neutral, acidic and optimized acidic (+ oxidizing agent) fly ash leaching have been investigated. Fly ash is characterized by refractory particles (Al-foil, unburnt carbon, quartz, feldspar) and newly formed high-temperature phases (glass, gehlenite, wollastonite) surrounded by characteristic dust rims. Metals are carried along with the flue gas (Fe-oxides, brass) and are enriched in mineral aggregates (quartz, feldspar, wollastonite, glass) or vaporized and condensed as chlorides or sulphates. Parameters controlling the mobilization of neutral and acidic fly ash leaching are pH and redox conditions, liquid to solid ratio, extraction time and temperature. Almost no depletion for Zn, Pb, Cu and Cd is achieved by performing neutral leaching. Acidic fly ash leaching results in depletion factors of 40% for Zn, 53% for Cd, 8% for Pb and 6% for Cu. The extraction of Pb and Cu are mainly limited due to a cementation process and the formation of a PbCu0-alloy-phase and to a minor degree due to secondary precipitation (PbCl2). The addition of hydrogen peroxide during acidic fly ash leaching (optimized acidic leaching) prevents this reduction through oxidation of metallic components and thus significantly higher depletion factors for Pb (57%), Cu (30%) and Cd (92%) are achieved. The elevated metal depletion using acidic leaching in combination with hydrogen peroxide justifies the extra effort not only by reduced metal loads to the environment but also by reduced deposition costs.

Derivatization technique to increase the spectral selectivity of two-dimensional Fourier transform infrared focal plane array imaging: analysis of binder composition in aged oil and tempera paint.

The interpretation of standard Fourier transform infrared spectra (FT-IR) on oil-based paint samples often suffers from interfering bands of the different compounds, namely, binder, oxidative aging products, carboxylates formed during aging, and several pigments and fillers. The distinction of the aging products such as ketone and carboxylic acid functional groups pose the next problem, as these interfere with the triglyceride esters of the oil. A sample preparation and derivatization technique using gaseous sulfur tetrafluoride (SF4), was thus developed with the aim to discriminate overlapping signals and achieve a signal enhancement on superposed compounds. Of particular interest in this context is the signal elimination of the broad carboxylate bands of the typical reaction products developing during the aging processes in oil-based paints, as well as signal interference originating from several typical pigments in this spectral range. Furthermore, it is possible to distinguish the different carbonyl-containing functional groups upon selective alteration. The derivatization treatment can be applied to both microsamples and polished cross sections. It increases the selectivity of the infrared spectroscopy technique in a fundamental manner and permits the identification and two-dimensional (2D) localization of binder components in aged paint samples at the micrometer scale. The combination of SF4 derivatization with high-resolution 2D FT-IR focal plane array (FPA) imaging delivers considerable advances to the study of micro-morphological processes involving organic compounds.

The molecular environment of phosphorus in sewage sludge ash: implications for bioavailability.

Producing a P fertilizer from sewage sludge ash (SSA) is a strategy to efficiently recycle P from a secondary raw material. The P speciation in four SSAs was characterized before and after the removal of heavy metals by a thermo-chemical treatment that involved CaCl addition. We chose complementary techniques to determine the direct P speciation, including X-ray powder diffraction, solid-state P direct-polarization magic-angle spinning nuclear magnetic resonance, and X-ray absorption near edge structure. Results from these techniques were compared with operational and functional speciation information obtained from a sequential P extraction and a plant biotest with Italian ryegrass grown on a soil-sand mixture with little available P. The speciation of P in untreated and thermo-chemically treated SSAs depended on their elemental composition. At a molar ratio of Ca:P ≤ 2, SSAs contained combinations of polymorphs of AlPO, ß-tricalcium phosphate, and apatite-like P species. In SSAs with a molar ratio of Ca:P > 2, an apatite-like molecular environment was predominant. The thermo-chemical process induced an increase in crystalline phases and enhanced the crystallinity of the P species. The structural order of the bulk sample was the most decisive parameter in controlling the P availability of the studied SSAs to plants. We conclude that, to produce a high-quality fertilizer and despite of the successful heavy metal removal, the thermo-chemical process requires further development toward enhanced P bioavailability.

Microcrystals coating the wing membranes of a living insect (Psocoptera: Psyllipsocidae) from a Brazilian cave.

Two specimens of Psyllipsocus yucatan with black wings were found with normal individuals of this species on guano piles produced by the common vampire bat Desmodus rotundus. These specimens have both pairs of wings dorsally and ventrally covered by a black crystalline layer. They did not exhibit any signs of reduced vitality in the field and their morphology is completely normal. This ultrathin (1.5 µm) crystalline layer, naturally deposited on a biological membrane, is documented by photographs, SEM micrographs, energy dispersive spectroscopy (EDS) and X-ray diffractometry (XRD). The crystalline deposit contains iron, carbon and oxygen, but the mineral species could not be identified. Guano probably played a role in its formation; the presence of iron may be a consequence of the excretion of iron by the common vampire bat. This enigmatic phenomenon lacks obvious biological significance but may inspire bionic applications. Nothing similar has ever been observed in terrestrial arthropods.

Assessing the Cr(VI) reduction efficiency of a permeable reactive barrier using Cr isotope measurements and 2D reactive transport modeling.

In Thun, Switzerland, a permeable reactive barrier (PRB) for Cr(VI) reduction by gray cast iron was installed in May 2008. The PRB is composed of a double array of vertical piles containing iron shavings and gravel. The aquifer in Thun is almost saturated with dissolved oxygen and the groundwater flow velocities are ca. 10-15m/day. Two years after PRB installation Cr(VI) concentrations still permanently exceed the Swiss threshold value for contaminated sites downstream of the barrier at selected localities. Groundwater δ(53/52)Cr(SRM979) measurements were used to track Cr(VI) reduction induced by the PRB. δ(53/52)Cr(SRM979) values of two samples downstream of the PRB showed a clear fractionation towards more positive values compared to four samples from the hotspot, which is clear evidence of Cr(VI) reduction induced by the PRB. Another downstream sample did not show a shift to more positive δ(53/52)Cr(SRM979) values. Because this latter location correlates with the highest downstream Cr(VI) concentration it is proposed that a part of the Cr(VI) plume is bypassing the barrier. Using a Rayleigh fractionation model a minimum present-day overall Cr(VI) reduction efficiency of ca. 15% was estimated. A series of 2D model simulations, including the fractionation of Cr isotopes, confirm that only a PRB bypass of parts of the Cr(VI) plume can lead to the observed values. Additionally, the simulations revealed that the proposed bypass occurs due to an insufficient permeability of the individual PRB piles. It is concluded that with this type of PRB a complete and long-lasting Cr(VI) reduction is extremely difficult to achieve for Cr(VI) contaminations located in nearly oxygen and calcium carbonate saturated aquifer in a regime of high groundwater velocities. Additional remediation action would limit the environmental impact and allow to reach target concentrations.

Assessment of long-term performance and chromate reduction mechanisms in a field scale permeable reactive barrier.

An innovative full-scale implementation of a permeable reactive barrier, consisting of a double-row of cylinders filled with zerovalent iron shavings, for chromate remediation was monitored over four years. Solid samples were analyzed to elucidate (i) the relevant corrosion mechanisms and products, (ii) the pathways of chromate reduction and immobilization, and (iii) the long-term performance of the barrier situated in a hydrological and geochemical complex groundwater regime. Sampling and analysis of groundwater and reactive material revealed an oxidative iron corrosion zone evolving in the inflow and a zone of anaerobic iron corrosion in the center and outflow of the barrier. Chromate reduction was mainly confined to the inflow region. The formation and thickness of corrosion rinds depended on sampling time, position, and depth, as well as on the size, shape, and graphite content In the inflow, the corrosion rinds mostly consisted of goethite and ferrihydrite. X-ray absorption fine structure spectroscopy revealed two distinct Cr(III) species, most likely resulting from homogeneous and heterogeneous redox reaction pathways, respectively. The longevity and long-term effectiveness of the PRB appears to be primarily limited by reduced corrosion rates of the ZVI-shavings because of the thick layers of Fe-hydroxides.